I am sorry that your dog is having problems. This sounds very much like a spinal cord issue. There could be a herniated disc, a growth/mass in the vertebra or spinal cord itself, or possibly an injury to the vertebra or spinal cord. Of course there are other potential causes, but these would be on the top of my list.
My recommendation would be to go to you veterinarian for a good exam and some xrays. They may even recommend some advanced testing like MRI or CT if that is a route you would wish to persue. Sometimes these problems can be helped with rest and appropriate treatment. If the problem has progressed to no deep pain response (a test your veterinarian will perform in their exam) and has been that way for greater than 24hrs, there will be little chance of any function being regained.
If your girl is pain free, eating, drinking, able to go to the bathroom, and otherwise healthy except for the paralysis, and there is no treatment to improve her condition, there are carts that support the rear end and allow them to get around. If this is a bigger issue (cancer, painful fracture, etc.), you will have to discuss prognosis and the best course of action with you veterinarian.
I hope this information has been helpful, and I wish you the best of luck,